1. Field of the Invention
The present invention relates to the managing of lamp fault conditions in electronic ballasts for one or more gas discharge lamps.
2. Description of the Related Art
Electronic ballasts include an inverter, typically a half-bridge, for powering gas discharge lamps. The inverter provides a square wave output voltage, which switching frequency is imposed by the lamp controller. The square wave output voltage is processed by a resonant output circuit that provides low current to warm the filaments (high switching frequency), high voltage to ignite the lamps (shift from high to low switching frequency) and a controlled current to power the lamps. Phase sequences and management is driven by the lamp controller.
In electronic ballasts, protection circuits are implemented in order to protect the lamp from damage due to excessive voltage, current, and heat. When a fault condition occurs, the electronic ballast is shut down or shifted to a different mode of operation. Because spurious electrical noise or momentary variation in the lamp current or in the lamp characteristics may be mistakenly interpreted as a lamp fault condition, the electronic ballast would be shut down or shifted to a different mode of operation unnecessarily. Further, if the lamp does not ignite on the first attempt, the status is treated as a lamp fault condition. This fault condition does not consider that lamps under low temperature often ignite after repetitive ignition phases. Existing ballasts address this problem by employing xe2x80x9cflasherxe2x80x9d type protection circuits that periodically attempt to ignite the lamps. Flasher type circuits provide an indefinite number of ignition attempts and are therefore potentially useful for low-temperature starting. Unfortunately, flasher type protection circuits often produce sustained repetitive flashing in one or more lamps, a characteristic that has proven to be an annoyance to users/occupants. Old lamps are hard to ignite too, so only one ignition attempt could be insufficient to ignite the lamp.
In an electronic ballast there is the necessity to detect real fault conditions in different lamp phases (preheating, ignition and running phase). To have a more precision in detection, the protection circuits need a determined sensitivity corresponding to the phases to monitor.
All these additional functions have to be implemented in reduced dimensions and using few external components.
The patent U.S. Pat. No. 5,969,483 discloses a method for management of fault conditions. It offers immunity to electrical noise and disturbances, and provides multiple ignition attempts for igniting the lamps under low temperature conditions avoiding flashing of the lamps. This method consists in repeating preheating phase and frequency shift, whenever a lamp fault occurs. Because the preheating phase is usually long, the fault management action could be slow.
In view of the state of the art described, an embodiment of the present invention provides a circuit able to avoid the drawback of the prior art.
An embodiment of the present invention is a method for fault management of electronic ballast for at least one gas discharge lamp comprising the steps of: preheating the lamp filaments by applying a low current for a predetermined time; igniting the lamp by increasing at a predetermined increasing rate the voltage applied up to a predetermined strike value; monitoring the lamp current; repeating the steps of igniting the lamp and monitoring the lamp current for a predetermined numbers of times if the lamp current is over a predetermined threshold; and powering the lamp at normal operating conditions.